... Planets, moons, asteroids and comets contain resources such as water and minerals, valuable to astronauts and potentially to Earth, with other elements such as platinum as a byproduct of the extraction. ... Lewis hoped we could defray cost s b y "bringing inexpensive products down to Earth. ... We could make travel to Mars cheaper by "bringing just enough fuel [to get there], then use the Martian CO2 atmosphere to go home. One ton of equipment will be able to produce 100 tons of fuel." ...